1. Field of the Invention
The present invention relates to a process for continuously preparing polyolefins having widely distributed molecular weights, and more particularly to a process for continuously preparing polyolefins having widely distributed molecular weights employing a highly active Ziegler type catalyst comprising a transition metal compound supported on a solid carrier and an organometallic compound and by the use of a plurality of reactors, each of the first and second stage reactors being kept in a condition under which desired polymerization reaction takes place.
2. Prior Art
Polyolefins commonly used for forming various molded articles, e.g. bottles, cable tubes and very thin films, must be fairly adapted to the molding conditions when they are brought to plasticized states and must be easily molded in desired shapes. Polyolefins having high melt indices, i.e. those having low average molecular weights, have improved moldabilities or workabilities due to flow characteristics, but they are deteriorated in their mechanical strengths, such as impact strength and tensile strength. On the other hand, polyolefins having low melt indices are improved in their strengths, but their moldabilities are poor. It is well known in the art that this contradictory problem can be solved by using polyolefins having widely distributed molecular weights.
In recent years, properties required of polyolefins become manifold, and there is a tendency of decreasing the amount of resins to be used as small as possible for saving resources as far as the required properties are satisfied. For instance, it has been tried to decrease the wall thicknesses of a bottle or of a film while retaining satisfactory strengths. Under these circumstances, there is an increasing demand for polyolefins which have good workabilities due to flow characteristics, high impact strengths, high tensile strengths and improved environmental stress cracking resistance, and which will give molded articles of good properties even when the amounts of the resins to be used are small.
Several processes for preparing polyolefins having widely distributed molecular weights wherein olefins are polymerized in multi-stage polymerization reactions, have been known. For example, such processes are disclosed in Japanese Kohkoku Pat. No. 42716/73 (Patent Publication No. 42716/73) and Japanese Kohkai Pat. No. 639/71 (Provisional Patent Publication No. 639/71). Each of these known processes comprises the first stage wherein the polymerization is effected by the use of a specific organometallic compound and in the presence of a large amount of hydrogen for forming relatively low molecular weight polymers, and the second stage wherein the polymerization is effected in the presence of a small amount of hydrogen for forming relatively high molecular weight polymers. However, this known process is disadvantageous in that an operation for separating and recirculating the hydrogen is required since the amount of hydrogen existing in the first stage is large. Moreover, the properties of the resin obtained by this process are not satisfactory in that a gelled mass tends to form in the molding step to result in poor molding qualities so that the strengths of the molded article are inferior.
Japanese Kohkoku Pat. No. 11349/71 Patent Publication No. 11349/71) discloses a process comprising the initial stage of effecting polymerization by the use of a predetermined amount of a specific polymerization catalyst in the presence of a small amount of hydrogen, and the subsequent stage of effecting polymerization in the presence of a large amount of hydrogen. However, this publication only discloses a discontinuous process, i.e. batch process, for preparing polyolefins, but not specifically referred to a continuous process for preparing polyolefins having widely distributed molecular weights, nevertheless the continuous process is more convenient from the industrial standpoint of view. In such a batch process for preparing a polyolefin, the first stage polymerization reaction is carried out under the predetermined condition in a single reactor in which a gas phase is normally existing at the upper portion thereof, and after the completion of the first stage reaction the condition is adjusted to suit for the second stage reaction, and then the second stage reaction is carried out in the same reactor in which a gas phase is also existing at the upper portion thereof. This process is disadvantageous from the industrial standpoint of view in that it requires complicated operations, that the production efficiency thereof is lowered and that difficulties are encountered in delicately controlling the reactions.
The present invention is particularly concerned with an industrially convenient process for continuously preparing polyolefins which have widely distributed molecular weights and superior properties by the use of a highly active Ziegler type catalyst comprising a transition metal compound supported on a carrier and an organometallic compound. The process for polymerizing olefins using the highly active Ziegler type catalyst has a very important advantage in that a step of removing the catalyst from the formed polymers can be avoided because of the fact that a large amount of polymers are formed with an extremely small amount of the catalyst. However, when polyolefins having widely distributed molecular weights are prepared with the use of the aforementioned highly active Ziegler type catalyst, and particularly when they are prepared by a generally known process other than the process of the present invention by the use of the multi-stage polymerization method which comprises an initial stage of forming relatively high molecular weight polymers and a successive stage of forming relatively low molecular weight polymers, the following difficulties will arise:
In order to prepare high molecular weight polymers in the initial or first polymerization stage, the first stage polymerization must be carried out in the absence of hydrogen or the concentration of hydrogen in the first stage should be low. As a result, a large amount of polymers is formed in an extremely short period of time due to the high activity of the catalyst. Particularly, if a gas phase mainly composed of a monomer is present in the polymerization reactor of the first stage, the concentration of the olefin monomer in the liquid phase is increased and it becomes difficult to control the monomer concentration in the liquid phase below the desired low level. With the increase in monomer concentration, the polymer formation is further accelerated to give polymers in short period of time that it becomes extremely difficult to control the reaction. It is, therefore, desirable to effect polymerization under a condition wherein the concentration of the monomer in the gas phase is low. However, the decrease in monomer concentration inevitably causes reaction pressure drop and necessitates provision of certain forced means such as a transfer pump for delivering the reaction product to the second stage, which means would cause fouling or blockage of the transfer passage.
Also, in the successive or second stage wherein relatively low molecular weight polymers are formed, the polymerization must be carried out with an increased concentration of hydrogen so that the polymer yield per unit time in the second stage is considerably decreased as compared to that in the first stage. Although the presence of a gas phase existing at the upper portion of the reactor and sufficiently enriched with the monomer is effective to increase the yield of polymers in the second stage, it is difficult to increase the monomer concentration to a satisfactory high level since a large amount of hydrogen is present in the polymerization reactor of the second stage. Further, even when a sufficient amount of the monomer is present in the second stage, the yield of polymers per unit time is decreased as compared to that obtainable in the polymerization reactor of the first stage.
If the yield of polymers in the first stage, i.e. the stage for forming high molecular weight polymers, is exceedingly increased and the yield of polymers in the second stage, i.e. the stage for forming low molecular weight polymers, is extremely decreased, the distribution of molecular weight of the resulting polymer is not sufficiently wide, and besides the polymer contains excessive amount of high molecular weight polymers, thus resulting in poor moldability.
In order to overcome the aforementioned disadvantages, it is advisable to use a smaller reactor in the first stage as compared to the polymerization reactor used in the second stage. However, when a small polymerization reactor is used in the first stage, the residence time of the reaction mixture in the first stage becomes too short to make the reaction condition uniform so that the reproducibility of the process might be badly affected. Furthermore, complicated operations are required for operating the small and large polymerization reactors mounted in line with one another in a continuous operation mode.